1. Technical Field
This application relates generally to fluid distribution. More specifically, the application provides apparatuses that enable fluid and sample particles to be uniformly distributed and collected to and from process channels, surfaces, or open volumes with minimal dispersion.
2. Prior Art
Many analytical and engineering processes use fluids to transport bands of material or samples between locations. In most, if not all, of these applications, it is important to keep spreading, skewing, and mixing of the bands to a minimum. As the materials move through the fluid handling systems, they can be subjected to dispersion resulting from diffusion, turbulent mixing, and non-uniform flow velocities. Unintended and undesirable hydrodynamic dispersion and stagnation can lead to material dilution, a blurring of the boundaries between adjacent bands, and unpredictable transit rates. Instead of distinct bands, there is a loss of separation resolution as materials arrive in a progressive manner over an extended period of time. In analytical and synthetic systems, it becomes difficult to reliably deliver precise concentrations of multiple reagents to specific locations at predetermined times and sequences. High throughput is most easily achieved when material or sample bands are closely spaced without significant dispersion during transport through the fluid handling system.
Such dispersion is often exacerbated when systems are miniaturized. There has been great interest in recent years in down-sizing analytical and reactor systems as the potential of micro- and nano-fluidic devices and technology to provide robust, highly accurate, high throughput, and low cost methodology has become more evident. “Lab-on-a-chip” platforms have been developed that enable sampling, separation, control, transport, mixing, incubation, reaction, and analysis, sometimes all within a single integrated device. As these systems become more complex, however, greater attention must be paid to possible sources of inadvertent band spreading.
In the late 1990s and early 2000s, a number of investigators studied turns and junctions (wyes and tees) in two-dimensional microscale systems and developed methods to minimize dispersion from these sources. Kopf-Sill and Parce (U.S. Pat. No. 5,842,787), for example, recommend using a channel turn geometry where the depth of the channel is greater that its width. The narrower width helps reduce dispersion caused by a difference in path length (and thus transit time) along the inner and outer walls of a turn, the so called “racetrack effect.” Further reduction in dispersion is suggested by fabricating the turn with the depth along the inner radius greater than that of the outer to reduce flow velocity along the inner radius. Microfluidic channels with variable depths, however, are difficult to manufacture and therefore are costly. Again recognizing the advantage of narrow channels, Paegel et al. (2000) recommend a “pinched turn” design in which the channel width is reduced prior to the turn and then expanded back to the original width once the turn is complete. In a similar manner, Griffiths and Nilson (U.S. Pat. No. 6,270,641) advise that the contraction and expansion regions actually be incorporated into the turn itself. This work is extended to include 45, 90 and 180 degree turns, wyes, and tees, as well as sample splitting devices and serpentine channels for folding long columns into small areas. Not all systems, however, require a dramatic alteration to the structure. Culbertson et al. (1998) points out that under certain circumstances the skewing of material caused by a turn can often be partially reversed by simply following the first turn with a second turn in the opposite direction. The extent to which the skewing can be reversed depends on the diffusivity of the particles in the particular fluid medium and the distance between the turns.
Another concern is the dispersion that can occur as fluid passes between regions of different cross-sectional areas in the flow system. Such transitions occur, for example, at inlet and outlet ports on separation channels or reaction chambers where the orifice at the ports may be orders of magnitude smaller than the width of the channel or chamber. As fluid medium with any entrained samples emerges from the inlet tube under laminar flow conditions, it fans out to the full width of the channel. The spreading fluid produces a crescent-shaped fluid front that persists as the fluid progressively moves along the length of channel. At the other end of the channel, the fluid medium and sample particles then funnel from across the width into the outlet tubing. As a consequence, sample particles follow flow paths of varying lengths through the channel. A particle on a direct line between the inlet and outlet along the center of the channel travels a significantly shorter distance than one that first diverges from the inlet to the channel's width before converging at the outlet end. The resulting path inequities lead to a spreading of sample particles away from each other and thus band broadening. Since the transitions between the different cross-sectional areas are generally at the ends of the channel or chamber, these flow non-uniformities and the resulting dispersion are often referred to as “end effects.”
Numerous suggestions have been proposed in the literature to lessen the influence of end effects on separation resolution. Most begin by incorporating a wedge-shaped (triangular) zone between the inlet or outlet port and the main body of the channel or chamber. With the port located at the apex of the triangle, the more gradual transition helps minimize the introduction of flow stagnation or turbulence into fluids moving through the transition. Using this design for example, Giddings et al. (1984) illustrated that the variation in sample path length through the channel and thus the dispersion can be reduced by simply minimizing the apex angle on the triangular transitional section. The work found reasonably good correlation between chromatographic theoretical plate height calculations and visual studies that examined the shape of the fluid flow profile using methylene blue samples in an optically transparent glass channel. In a second, more comprehensive investigation, Williams et al. (1986) mathematically corroborated the earlier work by employing conformal mapping techniques to generate theoretical equipotential curves, flow streamlines, and associated flow profiles as a function of apex angle.
Improvements in resolution, however, often come with tradeoffs and limitations. On close examination of the experimental and theoretical profiles, Williams et al. (1986) pointed out that, although reducing the apex angle decreased the end effect, it also unavoidably increased the relative contribution of the “edge effect” to the distortion of the fluid flow profile. The edge effect is a slowing of the fluid near the side walls of the channel. Photographic images of methylene blue samples in channels possessing large apex angles displayed the expected crescent-shaped flow profile compromised only by a small degree of tailing at the side walls of the channel, presumably due to edge effects. Since these tails incorporate only a very small fraction of the zonal material, their influence on the final flow profile is small. In channels with smaller apex angles, however, images showed significant tailing and departure from simple end effect calculated flow profiles. Williams et al. (1986) conjectured that the divergence could be attributed to the fact that the zonal material was in greater contact with the side walls for a longer distance. Edge effects generally become more prominent as the thickness of the channel increases. The ability to improve separation resolution by simply altering the apex angle therefore appears to be limited by the need to balance the deleterious consequences of the two (end and edge) effects. These studies by Giddings and Williams clearly show that reducing the apex angle in the transitional triangular section between regions of different cross-sectional areas is not by itself sufficient to eliminate the dispersion and resolution losses brought about by end effects.
Another method suggested for reducing end effects-associated band broadening was to reduce the volume of the transitional triangular section. From one perspective, the triangular sections can be thought of as being external to the main channel or chamber. Chromatographic theory has shown that the dispersion introduced by an external volume is proportional to the square of the volume. Two different approaches have been used to reduce the volume of the transitional section. The first was proposed by Giddings et al. (1984) and involved adjusting the thickness of the triangular section to about one-fourth that of a reference channel. This volume reduction work, which was done in conjunction with their apex angles studies, was performed using what might be considered a macroscale system. The channel had a width of 6 centimeters. Using a somewhat different approach, Sant et al. (2006) designed a flow system that incorporated an array of microstructural columns into the transitional sections. The logic here was that the columns would not only decrease the effective volume of the triangular sections, but would also redistribute the flow streams in a way that minimized differences in flow path lengths. To optimize results, the study examined a variety of microstructure geometries and configurations. Both experimental work and simulations were performed. This investigation, however, was done at a more microscale level using a channel with a width of only 3.5 millimeters. Interestingly, despite the size differential, both approaches resulted in decreased dispersion due to end effects and produced about a 50% reduction in the theoretical plate height when compared to their respective reference channels.
As indicated above, improvements in resolution, however, often come with tradeoffs and limitations. Sant et al. (2006) pointed out that further reduction in band broadening may ultimately be limited by increases in local edge effects that result from the presence of the microstructures. Consideration must also be given to the dramatically increased surface area provided by the columns and the increased possibility of intermolecular interaction with sample particles. Both approaches increase the complexity of the flow system and add another challenge to the manufacturing process. No commercially available flow system or instrument has incorporated either of the volume-reduction approaches.
Although the investigations by Giddings, Williams, and Sant were all conducted on channels employed in field-flow fractionation (FFF) separations, much of their work is adaptable to other techniques (both macro and micro) and helps illustrate the difficulties associated with designing devices to circumvent dispersion introduced by changes in cross-sectional area. Field-flow fractionation is a single-phase elution-based particle separation and characterization technique generally performed in a narrow, flat, rectangularly-shaped, ribbon-like, separation channel typically formed from two closely spaced parallel or concentric surfaces with inlet and outlet ports located at either end. The relatively simple configuration of the FFF channel and the extensive theoretical development of the FFF separation process readily facilitate the transfer of design considerations from FFF to other flow systems.
In 2006, Cummings and Fiechtner (U.S. Pat. No. 7,005,301) recommended an entirely different approach to dispersion reduction employing electrokinetic flow in microfluidic systems. Rather than directly addressing the inequality of flow path lengths associated with the racetrack and end effects as discussed above, their technique alters the direction of fluid flow by passing the flow across an abrupt interface between two regions possessing different specific permeabilities. Specific permeability is a quantity the authors define with dimensions of length, instead of the more commonly applied property of permeability, which has units of area. The system is constructed from a series of straight, open channels called “faceted flow prisms,” each possessing a predetermined specific permeability established by the depth and width of the channel. The ends of a channel are cut at predetermined angles. In connecting two channels together, the actual angle used on each channel is determined by the dimensions of the channel and the directional change to be brought about in the flow. By controlling the angle, interfacial contact area, and specific permeabilities of two connecting channels, the authors indicate that a wide range of turning angles and expansion ratios are possible with minimal dispersion. The system is modular in design, allowing the component prisms to be connected in an array of different configurations including transitions between regions of different cross-sectional areas.
The methodology developed by Cummings and Fiechtner is designed specifically for microfluidic applications and assumes the presence of ideal electrokinetic flow. Unlike pressure-driven flow which produces a parabolic profile and a velocity that depends on the size of channel, the velocity profile for electrokinetic (electroosmotic) flow is flat and the velocity is independent of the channel dimensions. Electrokinetic techniques, however, also exhibit some significant drawbacks. Electroosmotic flow is not particularly robust and is highly sensitive to the physicochemical properties of the solution and channel walls. When used with real samples, care must be taken to insure that solute molecules do not adsorb onto channel walls (often unavoidable and uncontrollable) creating inhomogeneities in surface charge density and local areas of flow anomalies. The need for an electrically conductive solution with tightly controlled pH and ionic strength generally makes the technique inapplicable to non-aqueous media or the use of solution gradients. Unfortunately, the composition of the conductive solution can be influenced by the electrochemical reactions at the system's electrodes that are used to maintain the required electric field in solution. High operating voltages (1-30 kV power supply) resulting in high currents in solution can also bring about runaway Joule heating and the need for cooling.
Other concerns about the faceted prism approach are its scalability and the manufacturing precision required to achieve the desired decrease in dispersion. Because each faceted interface introduces a small amount of dispersion, there is also a practical upper limit to the number of interfaces that can be coupled together. Using electrokinetic flow alone, it is difficult to extend the methodology for microscale channels to handle macroscale systems. Studies by Skulan et al. (2005), however, have found that velocity variations along faceted interfaces using pressure-driven flow can be unacceptable for many applications. Additional dispersion can be introduced by flow velocity inequities resulting from rounded corners and inexact interfacial angles and channel depths incurred during channel fabrication.
Although novel approaches have been developed to lessen the impact of end effects on band spreading, few of the methods are currently used on a regular basis, and none totally eliminate the problem. Without a solution, however, high resolution, high throughput separations and transfer processes are difficult to achieve. The problem, in fact, is becoming even more acute as new preparative scale applications move to wider channels to provide higher load capacity and analytical work trends towards smaller microscale systems to minimize analysis time and the amount of sample and fluid medium required. Since the relative contribution of end effects to separation inefficiency dramatically increases as the channel becomes either shorter or wider, end effects ultimately limit channel dimensions. It would therefore be of substantial interest and benefit to develop apparatuses with associated methodologies that would enable the uniform distribution and collection of materials and samples to and from process channels, surfaces and open volumes without the dispersion and resolution losses introduced by the end effects associated with transitions across regions of different cross-sectional area.
The present invention is directed to overcoming one or more of the problems and solving one or more of the needs as set forth above.